I don't think the team ever discussed it. I have personally never had a unicorn in my family so I did a bit of research when I found out about their abilities. It was on Carl's guide of course.
Another power that is very helpful in the gardening skill is Bless Flora, which will advance a plant to its next stage of growth. You can use this about three times with a max charge. Blessed plants have a chance to turn into Life Fruit. This spell will also revive a dead plant. Unicorns can also Curse Flora, which will kill a plant instantly. I am not sure why you'd want to use this, but you have the power!
This seems like a pretty powerful thing to have for an immortal dynasty.
Metro may decide that unicorns are allowed so long as you do not use their special plant skills.
Rachel